The Actual Finish indicates the date the task actually finished.
Once an Actual Finish is entered, the Completed Date is automatically updated and the % Completed is automatically updated to 100%. In addition, the task Duration can no longer be modified.
Note
Because Actual Start and Actual Finish override all other scheduling constraints, these dates should only be entered when the task has actually started or has been fully completed.
If Actual Start and Actual Finish are removed, either by changing the % Completed to zero or removing actuals, the task once again becomes subject to normal scheduling constraints.